This invention generally relates to systems and methods for cleaning the inner surface of tubular members and more particularly relates to a system and a method for abrading radioactive contaminants from the inside surface of a pipe.
A nuclear reactor power plant produces heat which is transferred to a liquid moderator that in turn circulates through at least one heat exchanger or steam generator for producing steam. The steam produced in the steam generator is then transferred to a turbine-generator for generating electricity in a manner well known in the art of nuclear power production. However, the circulating liquid moderator typically has dissolved and suspended radioactive solids therein. These highly radioactive solids usually are radioactive corrosion products formed from within the reactor, the plant piping system and the plant equipment through which the moderator circulates. During operation of the nuclear reactor, these radioactive corrosion products may form a sediment which will deposit on the inside surfaces of the plant piping system and plant equipment. In order to safely perform some types of maintenance (e.g., steam generator replacement) on the nuclear reactor power plant these highly radioactive deposits should be removed.
Of course, replacing a steam generator requires severing the end of the piping connected thereto and removing the steam generator. Thus, one end of the piping is exposed because it has been severed, and the other end of the piping is inaccessible because the other end of the piping remains connected to plant equipment. In any case, after the steam generator is removed, the radiation field emitting through the exposed pipe stubs, due to the radioactive sediments on the inside surface of the piping, may expose maintenance workers to an undesirable level of radiation. For example, the radiation field emitting through the pipe stubs may expose maintenance workers to the maximum allowable radiation dosage in only a few minutes of working time. Using many maintenance workers, wherein the radiation dose to any one worker is relatively low, is prohibitively expensive. Therefore, it is desirable to reduce the radiation dosage rate to a value that is as low as reasonably achievable in order to reduce the radiation exposure to maintenance workers who are performing the steam generator replacement. Cleaning of highly radioactive sediments from the inside surfaces of the piping, particularly sediments on the inside surface of the piping near the pipe stubs, will reduce the radiation exposure to maintenance workers performing the steam generator replacement. Moreover, it is important to suitably confine and remove the deposits cleaned from the inside surface so that the deposits are not transported through the piping to other reactor components where such deposits may contaminate and damage these other components. Therefore, one problem in the art has been to suitably remove the radioactive sediments from the inside surfaces of the piping so that the radiation fields emitting through the pipe stubs are reduced to a value that is as low as reasonably achievable. Another problem in the art has been to suitably confine and then suction the removed deposits and cleaning composition so that the removed deposits and cleaning composition are not transported through the piping to other reactor components.
A method of chemically decontaminating radioactive surfaces is disclosed by U.S. Pat. No. 3,615,817 issued Oct. 26, 1971, in the name of William T. Jordan, et al. entitled "Method of Decontaminating Radioactive Metal Surfaces". The Jordan, et al. patent discloses a method wherein decontamination is attained by employing a foam containing a chemical reagent which will chemically attack the radioactive contamination on the surface. Although the Jordan, et al. patent discloses a method of chemically decontaminating radioactive surfaces, this patent does not appear to disclose an apparatus for chemically decontaminating radioactive surfaces. Moreover, the Jordan, et al. patent does not appear to disclose either an apparatus or a method for mechanically decontaminating radioactive surfaces.
In general, chemical decontamination of nuclear power plant components, such as piping, is unacceptable for various reasons. For example, radioactive contamination is usually strongly bonded to the inner surface of the component; therefore, chemical decontamination may not adequately remove the contamination. In addition, transport of such chemicals to components other than the component being cleaned may lead to undesirable chemical attack and damage to other portions of the power plant. Therefore, another problem in the art has been to positively clean the inner surface of power plant components without the use of chemicals.
An apparatus for mechanically, rather than chemically, decontaminating a heat exchange tube is disclosed by U.S. Pat. No. 4,326,317 issued Apr. 27, 1982, in the name of Edward H. Smith, et al. entitled "Decontamination Apparatus" and assigned to the Westinghouse Electric Corporation. The Smith, et al. patent discloses a decontamination apparatus comprising a rotatable hone and brush capable of being inserted into a heat exchange tube of a nuclear steam generator for mechanically removing contamination from the inside of the heat exchange tube by contacting the inside of the tube. However, the Smith, et al. patent does not appear to disclose a system and method for decontaminating the inside surface of a tube using a dry abrasive grit and/or a liquid under pressure.
A device for mechanically cleaning instrumentation guide tubes using water under pressure is disclosed by U.S. Pat. No. 4,720,369 issued Jan. 19,1988, in the name of Gerard Cadaureille, et al. entitled "Device For Cleaning The Guide Tubes Of The Means of Measuring Neutron Fluxes in a Pressurized-Water Nuclear Reactor". The Cadaureille, et al. patent discloses a device for cleaning the instrumentation guide tubes of a pressurized-water nuclear reactor, comprising a pump for injecting demineralized water under pressure in the tubes to remove solid particles and an assembly for recovering the radioactive waste. Although the Cadaureille, et al. patent discloses a device for cleaning instrumentation guide tubes, the Cadaureille, et al. patent does not appear to disclose a system and method using a liquid-abrasive grit cleaning composition under pressure to clean the inside surface of the tube. Moreover, the Cadaureille, et al. patent does not appear to contemplate a system and method including seal means for confining the liquid-abrasive grit composition to a predetermined portion of the inside surface of the tube so that substantially all of the solid particles can be removed from the tube.
An apparatus for mechanically cleaning a pipe using a scraping device and a mixed flow of compressed air and abrasive material is disclosed by U.S. Pat. No. 4,622,709 issued Nov. 18, 1986, in the name of Shinichi Matsuda entitled "Cleaning Apparatus For Pipes". The Matsuda patent discloses a cleaning apparatus for a pipe which comprises at least one scraping device and a grinding device which is connected behind the scraping device. The scraping device has scraping segments which have scraping portions able to contact the inner surface of the pipe for removing deposits. The grinding device is provided for removing deposits which have not been removed by the scraping portions. The grinding device includes a rotatable ejection pipe terminating in a nozzle which ejects a mixed flow of compressed air and abrasive material such as sand, etc. for removing deposits; thus, the Matsuda patent does not appear to contemplate use of a liquid-abrasive grit composition to decontaminate the inner surface of the pipe. Further, although the Matsuda patent discloses a cleaning apparatus for pipes, the Matsuda patent appears to require both ends of the pipe to be accessible; thus, the Matsuda apparatus appears incapable of being used as intended when only one end of the pipe is accessible, such as is the case during the typical nuclear steam generator replacement. Moreover, although the Matsuda patent may disclose a scraping device and a grinding device for removing deposits from the inside surface of a pipe, the Matsuda patent does not appear to disclose seal means disposed near the scraping device and grinding device for suitably confining the removed deposits and abrasive grit material to a predetermined portion of the inside surface of the pipe so that substantially all of the deposits and abrasive grit material can be removed from the surface after the cleaning operation.
A sand-blasting and vacuum apparatus is disclosed by U.S. Pat. No. 4,333,277 issued June 8, 1982, in the name of Robert T. Tasedan entitled "Combination Sand-blasting and Vacuum Apparatus". The Tasedan patent discloses a sand-blasting apparatus which is operated by a vacuum system, wherein there is provided a sand-blasting gun having an air-inlet passage and a sand-inlet passage. Air is discharged into a mixing chamber, creating a vacuum therein to suck sand through the sand-inlet passage, and to discharge the sand through a nozzle under high-velocity so as to impinge against the work surface to be sanded. The sand is captured within the enclosed discharge end of the gun and then returned to the sand-storage tank. According to this patent, the sand is captured by a cup-like housing formed from resilient material provided with a resilient annular-seal member to be pressed against the work surface for capturing used sand and for allowing the used sand to be sucked back to the sand-storage tank. Although the Tasedan patent discloses a sand-blasting apparatus, the Tasedan patent does not appear to contemplate use of a liquid-abrasive grit composition for cleaning the work surface. Moreover, although the Tasedan patent discloses a cup-like housing provided with a resilient annular-seal member to be pressed against the work surface, the Tasedan patent does not appear to disclose seal means engaging the entire periphery of the inside surface of a pipe for confining a liquid-abrasive grit composition to a predetermined portion of the inside surface of the pipe.
Consequently, although the patents recited hereinabove disclose various apparatus and a method for cleaning the inner surface of tubular members, these patents do not appear to disclose a system and method for abrading radioactive contaminates from the inside surface of a tubular member, wherein the system includes suitable seal means for confining the abrasive cleaning composition and removed deposits to a predetermined portion of the inside surface of the tubular member so that substantially all of the contaminants and cleaning composition can be removed from the surface after the cleaning operation.
Therefore, what is needed is a system and a method for abrading radioactive contaminants from the inside surface of a tubular member, wherein the system includes suitable seal means for confining a liquid-abrasive grit cleaning composition and contaminants to a predetermined portion of the inside surface of the tubular member so that substantially all of the contaminants and cleaning composition can be removed from the surface after the cleaning operation.